The present disclosure relates to the subject matter contained in Japanese Patent Application No. 2001-311869 filed on Oct. 9, 2001, Japanese Patent Application No. 2002-064162 filed on Mar. 8, 2002, and Japanese Patent Application No. 2002-220100 filed on Jul. 29, 2002, which are incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor, processes for producing the same, a process cartridge, and an electrophotographic apparatus.
2. Description of the Related Art
Electrophotography is utilized in electrophotographic apparatus such as copy duplicator and laser beam printers because it is capable of high-speed high-quality printing. Recently, organic photoreceptors employing a photoconductive organic material have come to be mainly used as photoreceptors for such electrophotographic apparatus. In addition, the constitution of photoreceptors is shifting to the function-separated type in which a charge-generating material and a charge-transporting material are dispersed in separate layers (a charge-generating layer and a charge transport layer).
Many photoreceptors of such function-separated type have an undercoat layer interposed between the substrate and the photosensitive layer so as to prevent charge injection from the substrate into the photosensitive layer or for another purpose. Since properties of the photoreceptor, such as stability to cycling and environmental stability, depend not only on the charge-generating layer and charge transport layer but also on the properties of the undercoat layer, there is a desire for an undercoat layer which attains reduced charge accumulation during repetitions of use. An undercoat layer further plays an important role in preventing image quality defects. It is highly effective to form an undercoat layer in order to diminish image quality defects attributable to defects or fouling of the substrate or to coating film defects or unevenness of an overlying layer, e.g., the charge-generating layer.
In recent years, contact electrification type charging units reduced in ozone generation have come to be used in place of corotrons as the charging units of electrophotographic apparatus. However, when a contact charging unit is used, the photoreceptor is apt to be charged unevenly. Furthermore, in case where the photoreceptor has a local deteriorated area, a local high electric field is applied to the deteriorated area during contact charging to cause an electrostatic pinhole, which tends to result in an image quality defect. Although this pinhole leakage can occur due to coating film defects of the photosensitive layer, it may also occur because electrically conducting paths are apt to be formed by electroconductive foreign particles (e.g., carbon fibers or carrier particles) which have generated within the electrophotographic apparatus and are in contact with the photoreceptor or have penetrated into the photoreceptor.
Under these circumstances, investigations have been made on constituent materials for undercoat layers and properties of the layers so as to avoid those phenomena accompanying the use of a contact charging unit, and electrophotographic photoreceptors having various undercoat layers have been proposed. For example, Japanese Patent Laid-Open No. 204641/1986 discloses an electrophotographic photoreceptor having an undercoat layer which contains a permittivity regulator and thereby has a volume resistivity and a permittivity in respective given ranges. Japanese Patent Laid-Open No. 113758/1989 discloses an organic photoreceptor having an undercoat layer comprising a binder resin, a charge-transporting material, and electroconductive fine particles. Furthermore, Japanese Patent Laid-Open No. 84393/1995 discloses an electrophotographic photoreceptor having an undercoat layer which contains compact particles of fine cicular titanium oxide particles and has a given value of volume resistivity.
However, even with any of those electrophotographic photoreceptors of the related art, it has been extremely difficult to obtain sufficient image quality when they are used together with a contact charging unit. The reasons for this are as follows. From the standpoint of the property of preventing charge leakage due to pinhole generation or the like (hereinafter referred to as xe2x80x9cleakage preventive propertiesxe2x80x9d), it is desirable that the thickness of the undercoat layer be large (e.g., about from 10 to 30 xcexcm). For obtaining sufficient electrical properties, it is necessary to reduce the resistance of the undercoat layer having such an increased thickness. As a result, however, that blocking properties of the undercoat layer by which charge injection from the substrate into the photosensitive layer is prevented become insufficient and fogging is hence apt to occur.
On the other hand, investigations are being made on processes for forming a photoreceptor which comprises an electroconductive support layer (substrate) and formed thereon a layer containing electroconductive fine particles in order to attain stable electrical properties by diminishing the increase in residual potential while hiding the defects of the electroconductive support layer.
An example of such processes is proposed, for example, in Japanese Patent Laid-Open No. 45961/1991. In this process, a photoreceptor having an undercoat layer with a two-layer structure is produced. This process comprises forming a layer containing electroconductive fine particles on an electroconductive support layer, e.g., an aluminum substrate, and further forming a layer having the same constitution as usual undercoat layers on the layer containing electroconductive fine particles. In this process, the layer containing electroconductive fine particles is intended to hide defects, such as surface irregularities and fouling, of the electroconductive support layer and to regulate electrical resistance, while the layer having the same constitution as usual undercoat layers is intended to have a blocking function (inhibition of charge injection).
In another process is produced a photoreceptor having a constitution comprising an electroconductive support layer and formed thereon an undercoat layer which consists only of a layer containing electroconductive fine particles and combines the blocking function and resistance-regulating function. This type of photoreceptor and processes for producing the same are disclosed, for example, in Japanese Patent Laid-Open Nos. 258469/1997, 96916/1997, and 2001-75296.
However, the above-described electrophotographic photoreceptors of the related art are still insufficient in having electrical properties sufficient to enable the photoreceptors to withstand repetitions of use. These electrophotographic photoreceptors have had a problem that when they are repeatedly used, the residual potential increases and this results in fogging such as black spots on the image.
Specifically, the electrophotographic photoreceptor having an undercoat layer of a two-layer structure disclosed in Japanese Patent Laid-Open No. 45961/1991 has had the following problems. It has poor leakage preventive properties and is hence apt to suffer the pinhole leakage described above. Because of this, the photoreceptor comes to have reduced electrification characteristics and causes a decrease in image density with repetitions of use. Another drawback of this photoreceptor is that due to the two-layer structure, the photoreceptor production is troublesome and costly.
The electrophotographic photoreceptors disclosed in Japanese Patent Laid-Open Nos. 258469/1997, 96916/1997, and 2001-75296 are advantageous in that because the undercoat layer has a single-layer structure, the photoreceptor production processes can be simplified and the cost of the photoreceptors can be reduced. However, the necessity of forming a single layer combining a resistance-regulating function and a charge injection-inhibiting function imposes limitations in selecting constituent materials for the undercoat layer.
From the standpoint of preventing pinhole leakage by enhancing the leakage preventive properties of an undercoat layer, it is effective to increase the thickness of the undercoat layer (hereinafter often referred to as thickness increase). In order for an undercoat layer to have an increased thickness, it should have reduced electrical resistance so as to attain satisfactory electrical properties. However, a reduction in electrical resistance impairs the charge-blocking function and this tends to enhance the occurrence of image quality defects such as fogging. Undercoat layers having increased thicknesses further have problems that they are difficult to form and have insufficient mechanical strength. Furthermore, there has been a problem that an increase in undercoat layer thickness may result in a decrease in photoreceptor sensitivity.
Because of those problems, the thicknesses of undercoat layers containing electroconductive metal oxide particles, e.g., titanium oxide particles, have been in the range of about from 0.01 to 20 xcexcm at the most. For example, in Japanese Patent Laid-Open Nos. 258469/1997, 96916/1997, and 2001-75296, there is a description to the effect that it is undesirable to increase the thickness of the undercoat layer of the electrophotographic photoreceptor disclosed therein beyond 20 xcexcm for the reasons given above.
The invention has been achieved in view of the problems of the prior art techniques described above. An aim of the invention is to provide an electrophotographic photoreceptor which combines a high level of leakage preventive properties and a high level of electrical properties and which, even when used together with a contact charging unit, can attain satisfactory image quality without causing image quality defects such as fogging. Another aim of the invention is to provide a process for producing the electrophotographic photoreceptor. Still another aim of the invention is to provide a process cartridge and an electrophotographic apparatus each employing the electrophotographic photoreceptor.
A further aim of the invention is to provide a process for producing an electrophotographic photoreceptor which has high durability capable of sufficiently preventing electrical properties from decreasing with repetitions of use and further has high resolution quality. Still a further aim of the invention is to provide an electrophotographic photoreceptor obtained by the process, a process cartridge, and an electrophotographic apparatus.
The present inventors made intensive investigations in order to accomplish those aims. As a result, it has been found that those aims are accomplished with an electrophotographic photoreceptor comprising an electroconductive substrate, a photosensitive layer, and an interlayer formed therebetween which comprises fine metal oxide particles and a binder resin and has a volume resistivity and environmental dependence of volume resistivity which are within respective specific ranges when determined under given conditions. The invention has been completed based on this finding.
According to the first respect of the invention, an electrophotographic photoreceptor is provided which comprises an electroconductive substrate, an interlayer formed over the substrate, and a photosensitive layer formed over the interlayer, wherein the interlayer comprises fine metal oxide particles and a binder resin and the interlayer, when an electric field of 106 V/m is applied thereto at 28xc2x0 C. and 85% RH, has a volume resistivity of from 108 to 1013 xcexa9xc2x7cm and, when an electric field of 106 V/m is applied thereto at 15xc2x0 C. and 15% RH, has a volume resistivity which is not higher than 500 times of the volume resistivity thereof as measured when an electric field of 106 V/m is applied thereto at 28xc2x0 C. and 85% RH.
The electrophotographic photoreceptor of the invention has, interposed between the substrate and the photosensitive layer, an interlayer which comprises fine metal oxide particles and a binder resin and satisfies the requirements shown above concerning volume resistivity and its dependence on the environment. Due to this constitution, both of leakage preventive properties and electrical properties are sufficiently enhanced. Consequently, even when the electrophotographic photoreceptor is used together with a contact charging unit, it can attain satisfactory image quality without causing image quality defects such as fogging.
According to the second respect of the invention, a process for producing an electrophotographic photoreceptor is provided which comprises forming an interlayer and a photosensitive layer over an electroconductive substrate, wherein the interlayer, when an electric field of 106 V/m is applied thereto at 28xc2x0 C. and 85% RH, has a volume resistivity of from 108 to 1013 xcexa9xc2x7cm and, when an electric field of 106 V/m is applied thereto at 15xc2x0 C. and 15% RH, has a volume resistivity which is not higher than 500 times of the volume resistivity thereof as measured when an electric field of 106 V/m is applied thereto at 28xc2x0 C. and 85% RH, the interlayer being obtained by surface treating fine metal oxide particles with at least one coupling agent selected from the group consisting of silane coupling agents, titanate coupling agents, and aluminate coupling agents, heat-treating the surface treated fine metal oxide particles at 180xc2x0 C. or higher, adding the heat-treated fine metal oxide particles and a binder resin to a given solvent to obtain a coating fluid, applying the coating fluid to an electroconductive substrate, and drying the coating fluid applied.
According to the third respect of the invention, a process for producing an electrophotographic photoreceptor is provided which comprises forming an interlayer and a photosensitive layer over an electroconductive substrate, wherein the interlayer, when an electric field of 106 V/m is applied thereto at 28xc2x0 C. and 85% RH, has a volume resistivity of from 108 to 1013 xcexa9xc2x7cm and, when an electric field of 106 V/m is applied thereto at 15xc2x0 C. and 15% RH, has a volume resistivity which is not higher than 500 times of the volume resistivity thereof as measured when an electric field of 106 V/m is applied thereto at 28xc2x0 C. and 85% RH, the interlayer being obtained by surface treating fine metal oxide particles with a treating liquid comprising a given solvent and at least one coupling agent selected from the group consisting of silane coupling agents, titanate coupling agents, and aluminate coupling agents, heat-treating the surface treated fine metal oxide particles at a first heat treatment temperature, heat-treating at a second heat treatment temperature the fine metal oxide particles which have been heat-treated at the first heat treatment temperature, adding the fine metal oxide particles heat-treated at the second heat treatment temperature and a binder resin to a given solvent to obtain a coating fluid, applying the coating fluid to an electroconductive substrate, and drying the coating fluid applied.
By each of the processes according to the second and third respects described above, an interlayer satisfying the requirements shown above concerning volume resistivity and its dependence on the environment can be easily formed without fail due to the use of the fine metal oxide particles which have undergone a surface treatment with a given coupling agent and a heat treatment. As a result, the photoreceptor obtained has sufficiently enhanced leakage preventive properties and sufficiently enhanced electrical properties. Consequently, even when the photoreceptor is used together with a contact charging unit, it can attain satisfactory image quality without causing image quality defects such as fogging.
According to the fourth respect of the invention, a process cartridge is provided which comprises an electrophotographic photoreceptor and, united with the photoreceptor, at least one of a charging unit, a development unit, a cleaning unit, an erase unit, and a transfer unit, the electrophotographic photoreceptor comprising an electroconductive substrate, an interlayer formed over the substrate, and a photosensitive layer formed over the interlayer, wherein the interlayer comprises fine metal oxide particles and a binder resin and the interlayer, when an electric field of 106 V/m is applied thereto at 28xc2x0 C. and 85% RH, has a volume resistivity of from 108 to 1013 xcexa9xc2x7cm and, when an electric field of 106 V/m is applied thereto at 15xc2x0 C. and 15% RH, has a volume resistivity which is not higher than 500 times of the volume resistivity thereof as measured when an electric field of 106V/m is applied thereto at 28xc2x0 C. and 85% RH, the process cartridge being capable of being freely attached to and removed from the main body of an electrophotographic apparatus.
According to the fifth respect of the invention, an electrophotographic apparatus is provided which comprises an electrophotographic photoreceptor, a charging unit which charges the electrophotographic photoreceptor, an exposure unit with which the electrophotographic photoreceptor charged by the charging unit is exposed to light to form an electrostatic latent image, a development unit which develops the electrostatic latent image with a toner to form a toner image, and a transfer unit which transfers the toner image to a receiving medium, the electrophotographic photoreceptor comprising an electroconductive substrate, an interlayer formed over the substrate, and a photosensitive layer formed over the interlayer, wherein the interlayer comprises fine metal oxide particles and a binder resin and the interlayer, when an electric field of 106 V/m is applied thereto at 28xc2x0 C. and 85% RH, has a volume resistivity of from 108 to 1013 xcexa9xc2x7cm and, when an electric field of 106 V/m is applied thereto at 15xc2x0 C. and 15% RH, has a volume resistivity which is not higher than 500 times of the volume resistivity thereof as measured when an electric field of 106 V/m is applied thereto at 28xc2x0 C. and 85% RH.
The process cartridge and electrophotographic apparatus of the invention each have a contact charging unit. However, the use of this contact charging unit in combination with the electrophotographic photoreceptor of the invention reconciles a high level of leakage preventive properties with a high level of electrical properties. Consequently, the effect that satisfactory image quality is obtained without causing image quality defects such as fogging is produced, although it has been extremely difficult to attain this effect with any of the usual process cartridges and electrophotographic apparatus having a contact charging unit.